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Shopmaster blows breaker MIG only

11-27-2013, 12:22 PM

I have a Miller Shopmaster 300, s/n KD408963. My manual is January 1993.

It has worked fine until recently. Now it will sometimes mig weld, sometimes it pops the breaker. The breaker pops right when I pull the trigger, before the wire strikes an arc. I looked at my liner, it had a couple of minor kinks so I replaced it, no change in symptoms. I was welding 1/8" steel, it will weld fine for a bit, pause, pull the trigger again and the breaker pops.

Stick welding (reverse polarity, 170A) works fine.

I am using a 50A breaker which has always worked for me. I changed to the 50A circuit for my plasma cutter (different breaker), same problem.

Comment

Probably going to find that one is loose and blown up. Though if one is loose, most of the others are loose as well. You can mark the +& - on the rails and meter (capacitance) on each one. Change out the ones that fail, but rest assured the ones you didn't replace will fail in a short time frame. Don't nessessarily have to buy them from Miller. Just have to buy the same style and same specs

Comment

I don't follow what you mean by "trigger to conduit short". Are you saying the trigger wires could be shorting inside the gun/cable?

I did take all the caps out (after marking the bus bars for +/-) and test them individually. I hooked them up to a 40V power supply (all I had) with 50 Ohm series resistor. I turned on the power and watched the voltage climb. All caps took about 10 seconds to reach 38V, which is pretty close.

Next I could try putting them in the machine one at a time. I don't need to weld for the problem to occur, just pulling the trigger is enough.

Steve

Comment

don't know about the way you check capacitance, also don't know if there is a heat problem with one of your caps. a capacitance test would tell me everything I need to know. Now there are 2 trigger wires in the gun cable sheath and likely one spare. You will want to test them against the copper current carrying conduit. If one is shorted it may cause a surge

Comment

The test I did checked the capacitance. I set up an R*C circuit, 50 Ohms and 30,000uF gives 1.5 seconds. I watched each cap charge for 10 seconds; they all got to about 38V (40V supply). I think they are all ok at 40V anyway.

I don't think temperature comes into the situation, it will pop the breaker on the first attempt some times, stone cold welder.

I also swapped the trigger wires to the spares. This did not help, unfortunately.